Method and apparatus for improving the viewing angle of an LCD screen by twisting the polarizers and compensating structures

ABSTRACT

A display device with a liquid crystal cell including a layer of twisted nematic liquid crystal placed between two polarizers and including, between the liquid crystal layer and each of the polarizers, a structure for compensating for cell contrast variations according to the observation angle which has an orientation in the plane of the cell. The pass directions of the polarizers are separated by an angle of 90+A degrees, in which A is not zero, and the orientations of the two compensation structures are separated by an angle of 90+B degrees, in which B is not zero. Such a structure may find particular application to a liquid crystal screen, especially for avionics.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to electrooptical display devicesmodulating the light passing through them and more specifically toliquid crystal panels.

2. Discussion of the Background

These panels exhibit promising characteristics for making displayscreens in avionics, in that they are less bulky than the conventionalcathode tube screens and they use less power.

On a liquid crystal panel, an image is displayed using juxtaposedcolored, or black, elementary dots. An elementary dot corresponds to thelight transmitted to its front face by a liquid crystal cell illuminatedon its rear face. From the rear to the front, a liquid crystal cellgenerally comprises a stack of a polarizer, a first transparentsubstrate, a thin layer of liquid crystal, a second transparentsubstrate and an analyser.

These transparent substrates comprise electrodes which are alsotransparent, to which the application of voltage makes it possible tosubject the liquid crystal molecules to an electric field perpendicularto the plane of the cell.

A liquid crystal molecule also exhibits two remarkable characteristics;on one hand, it is capable of modifying the polarization of lightpassing through it, depending on said molecule's spatial orientationand, on the other hand, an electric field is capable of changing itsorientation.

Thus, the rest state (zero voltage) of and the application of voltage tothe liquid crystal layer lead to two different arrangements of theliquid crystal molecules in the layer, defining two states (activatedstate and unactivated state) of the cell such that, for example, in onestate the cell allows light to pass and in the other the cell absorbsit. Depending on its state, the cell makes it possible to display awhite dot or a black dot. Gray dots can be produced with intermediatevoltages which impose other orientations on the liquid crystalmolecules. In addition, the insertion of a color filter into the cellstack makes it possible to present a colored dot. This technology makesit possible to display images in black and white or in color.

However, with a liquid crystal panel, the image perceived depends on theviewing angle at which the panel is observed. A high-quality imageperceived by the observer when he is looking along the direction normalto the plane of the panel is distorted when he is looking in a directioninclined with respect to this normal direction. It is this whichgenerally limits the use of a liquid crystal panel to observationdirections which depart little from the normal to the panel, i.e. thepanel has a restricted viewing angle.

For a display screen which must be legible to an observer whose positionis not fixed and/or to several observers placed around the screen, suchas, for example, a screen in an airplane pilot's cabin, the restrictionin the viewing angle of the liquid crystal panel is a serious drawback.

For the screen user, the liquid crystal layer exhibits defects inoptical behaviour. The defects are especially due to the light contrastbetween the states of a cell, which contrast causes the particularproblem of changing with the observation angle and therefore ofdisturbing the observation of a panel.

This effect is explained by the natural birefringence of a liquidcrystal molecule in which the modification by the molecule of thepolarization of the light passing through it depends on the relativeorientation between this light and the molecule, the change ofobservation angle leads to a modification of the polarization of thelight received by the analyser and therefore to a modification of thetransmission of the cell.

The prior art provides partial corrections of this birefringence incertain situations where it is a problem for various types of liquidcrystal cells.

We are more particularly interested in cells comprising a twistednematic liquid crystal and crossed polarizers (one polarizer, oneanalyser), located on either side of the liquid crystal layer. The helixrotates the polarized light by about 90 degrees. In their unactivatedstate, the cells strongly transmit the light received. Their activatedstate corresponds to considerable absorption of the light; an activatedcell observed along its normal has a very low light transmission. Themain defect of these cells is, in the activated state, a marked increasein the light transmission for observation inclined with respect to thenormal to the cell. A black dot observed perpendicularly to the cellbecomes clearer when the observer moves away from the normal to thecell, the contrast between white and black decreases with this movementwith respect to the normal. The contrast is the ratio of thetransmissions of each state that is the ratio of the cell transmissionin the activated state to the cell transmission in the unactivatedstate.

In the activated state, the birefringence of the liquid crystalmolecules is undesirable.

The prior art proposes to correct, however incompletely, thisbirefringence by the addition of a birefringent compensation film intothe cell stack.

A first known correction consists in using a uniaxial birefringent filmexhibiting a negative anisotropy of refractive index in the directionperpendicular to the plane of the cell. The film is negative uniaxialwith an extraordinary optical axis normal to the cell.

Corrections giving increased satisfaction were then developed. Thus,Patent EP 0,646,829 proposes a birefringent film comprising a supportexhibiting the characteristics of the first known correction on which adiscotic liquid crystal oriented by rubbing is polymerized. It describesa type of birefringent film comprising two negative uniaxial media, eachexhibiting an extraordinary optical axis, one medium being parallel tothe normal to the cell, the other medium being inclined with respect tothis normal on one hand and with respect to the plane of the cell on theother.

With compensation films, the problem consists in further widening thefield of observation of a liquid crystal panel in a plane perpendicularto the panel. In particular, for a panel placed vertically, it isendeavored to improve the horizontal viewing angle ensuring goodlegibility of the panel for an observer moving horizontally to the rightor to the left of the panel.

SUMMARY OF THE INVENTION

The invention provides a novel solution which consists in uncrossing thepolarizers and the compensation films in order to widen the observationangle.

More specifically, the invention provides a display device with a liquidcrystal cell comprising a layer of twisted nematic liquid crystal placedbetween two polarizers and comprising, between the layer of liquidcrystal and each of the polarizers, a structure for compensating for thecell contrast variations according to the observation angle, whichstructure comprises a layer of a negative uniaxial birefringent materialwith an extraordinary axis perpendicular to the plane of the cell onwhich a layer of a negative uniaxial birefringent material with anextraordinary axis inclined with respect to the normal to the plane ofthe cell is superimposed, the orientation of the compensation structurebeing parallel and in the opposite direction to the projection of theextraordinary axis inclined to the normal to the plane of the cell,characterized in that the pass directions of the polarizers areseparated by an angle of 90+A degrees where A is not zero and theorientations of the two compensation structures are separated by anangle of 90+B degrees where B is not zero.

Compensation of the cell contrast variations according to theobservation angle corrects the irksome effects of transmissionvariations of the birefringent cell according to the observation angle.The compensation corrects the cell birefringence.

The novel solution of the invention consists in arranging the polarizersin a particular relative position which is different to that of theprior art in which the polarizers are crossed perpendicularly in such away that one polarizer can block the light polarized by the other. Theparticular position may for example be obtained by uncrossing one pairof polarizers of the prior art. The uncrossing corresponds to a rotationabout the normal to the cell of one of the polarizers and to a rotationabout the same axis, but in the opposite direction, of the otherpolarizer. The absolute values of the angles of each of the uncrossingrotations are equal or different.

The uncrossing is done in the direction where the increase in the rangeof viewing angles is the most favorable. The uncrossing will take placein one direction or in another depending on whether the nematic crystalhelix generated by the alignment directions of the molecules on thefaces of the liquid crystal layer, are a helix in the anticlockwise orclockwise direction.

The uncrossing is described in order to facilitate understanding of therelative positions in the invention, but the relative arrangement of theelements of the cell may, of course, be obtained directly withoutphysically carrying out rotation operations.

The structure for compensating for the cell contrast variations ispreferably in the form of a plane film parallel to the plane of the celland characterized by an orientation direction in this plane. In theinvention, the orientation directions of each of the compensationstructures, one on each side of the liquid crystal layer, are no longerperpendicular, they are also uncrossed.

In a preferred embodiment of the invention, the absolute values of theangles of each of the uncrossing rotations are different, one preferablybeing zero.

The alignment directions of the molecules on the faces of the liquidcrystal layer are preferably perpendicular.

The uncrossing of the polarizers, as is described in order to facilitateunderstanding, is preferably done using polarizers which are crossedwith each other and are such that each polarizer is crossed with thealignment direction of the face closest to the liquid crystal layer.These relative orientations between a polarizer and the alignmentdirection of the closest liquid crystal molecules have the advantage oflimiting the contrast inversions.

The invention makes it possible to widen the observation field of aliquid crystal panel. In particular, for a vertical panel, thehorizontal viewing angle is widened.

Other characteristics and advantages of the invention will appear onreading the description which follows and which is made with referenceto the appended drawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically a liquid crystal cell in a knownconfiguration with crossed polarizers, with FIG. 1a illustrating theunactivated state of the cell and FIG. 1b illustrating the activatedstate of the same cell;

FIG. 2 shows a cell of the prior art with two optical compensationstructures;

FIG. 3 shows a part of a compensation film of the prior art;

FIG. 4 shows an end-on view of the orientations according to the priorart of the cell elements;

FIG. 5 shows an embodiment of the invention;

FIG. 6 shows another embodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A liquid crystal cell is shown schematicaly in FIG. 1; it comprises, ina known way in the stack along the Oz axis, a polarizer 16, a firsttransparent substrate 12, liquid crystal molecules 11, a secondtransparent substrate 14 and an analyser which is a polarizer 18 crossedwith the first polarizer 16. The first 12 and second 14 substrates andthe molecules 11 together forms a liquid crystal layer 10 which is planeand parallel to the Oxy plane.

In the case of a twisted nematic liquid crystal, the liquid crystalmolecules close to one of the two transparent substrates, 12, areoriented in a first alignment direction, for example parallel to the Oxaxis, and the molecules close to the other substrate, 14, are orientedalong a second alignment direction, for example parallel to the Oy axis.The first and second alignment directions are preferably perpendicularto each other.

The alignment can be obtained by treating those faces of the substrates12 and 14 which are in contact with the liquid crystal, for example byrubbing the surface in the alignment direction.

When the cell is in the rest state, it is in the unactivated stateillustrated by FIG. 1a in which the molecules 11 of the crystal layerare parallel to the Oxy plane of the cell and form, using theperpendicular alignment directions of the substrates, a helix within thethickness of the layer 10. Light received on the rear face of theunactivated cell is linearly polarized by the polarizer 16, then, onpassing through the thickness of the twisted liquid crystal layer 10,this linear polarization is modified by the birefringence of themolecules 11 that it passes through and undergoes a rotation of 90degrees due to the helix. On exiting from the layer 10, the light ispolarized perpendicular to the first polarizer 16 and it can thereforepass freely through the second crossed polarizer 18, the pass directionof which is perpendicular to that of the first polarizer 16. The cell inthe unactivated state strongly transmits light.

When a voltage is applied to the cell, it is in the activated stateillustrated by FIG. 1b in which an electric field perpendicular to thethin layer 11 is applied between the energized transparent electrodesborne by the substrates 12 and 14. As the molecules 11 of the layer 10tend to align with the electric field, they stand up in relation to theplane of the cell and the previously observed helix is destroyed withinthe thickness of the layer 10. In the activated state, the light alsopasses through the layer 11 but the arrangement of molecules that itencounters means that it tends to maintain a plane polarization in thedirection imposed by the polarizer 16. The light at the exit of thelayer 10 is absorbed by the second polarizer 18 crossed with the firstpolarizer.

In the activated state, the cell observed along the directionperpendicular to its plane transmits virtually no light. However, whenthe observation direction is inclined with respect to the normal to thecell, the inclined light is affected by the birefringence of themolecules which modifies its polarization and some of this light thenpasses through the second polarizer.

The arrangement of the molecules in the energized liquid crystal layeris quite complex, since the observed destruction of the helix in therest state is only partial.

When a liquid crystal molecule is stretched, to a first approximation,it behaves optically as a positive uniaxial birefringent medium whoseextraordinary axis is oriented along the length of the molecule. Theenergized layer of liquid crystal may be represented in a highlysimplified manner by a stack of sublayers in which the birefringence ishomogeneous and the molecules have the same orientation. In thisrepresentative model, each sublayer is a positive uniaxial birefringentmedium, the direction of the extraordinary axis of which characterizesthe sublayer.

FIG. 2 shows, along the Z axis, the known stack of a liquid crystal cellbacklit by the light rays 30 and comprising two optical compensationfilms 25, 27, one on each side of the plane liquid crystal layer 20,said films being situated between the two polarizers 26 and 28. Theplane of the cell is the XY plane. The rear 25 and front 27 films are,for example, of the type of those described in Patent EP 0,646,829. Eachfilm 25, 27 comprises within its thickness a negative uniaxial medium22, 24 with extraordinary axis Z perpendicular to the XY plane of thecell and a negative uniaxial medium 21, 23 with extraordinary axisinclined with respect to the normal Z to the cell and with respect tothe XY plane of the cell. The inclined uniaxial medium 21, 23 being theclosest to the liquid crystal layer 20.

The polarizers 26, 28 of this cell are crossed-their pass directions,respectively P26 and P28, are perpendicular to each other. The crystallayer 20 has an alignment direction A31 of the molecules on its rearface 31 in the XY plane which is perpendicular to that A33 of the frontface 33, which is also in the XY plane. In the rest state, the crystallayer has a helix whose twist is, for example, 90 degrees within itsthickness. The pass direction P26, P28 of each polarizer is preferablycrossed with the alignment direction A31, A33 of the face closest to theliquid crystal layer.

The orientation of the uniaxial medium 21 adjacent to the rear face 31of the layer 20 is marked in the XY plane by the direction F21 ofrubbing carried out when preparing the film 25 according to the patentcited. The direction F21 is parallel to the alignment direction A31 ofthe molecules on the rear face 31 but in the opposite direction.Similarly, the orientation of the uniaxial medium 23 adjacent to thefront face 33 of the layer 20 is marked by the direction F23 paralleland in the opposite direction to the alignment A33 of the molecules ofthe front face 33, the direction F23 corresponding to the rubbing forpreparing the film 27. The directions F21 and F23 are perpendicular toeach other.

FIG. 3 shows part of the rear compensation film 25 already illustratedby FIG. 2; this film 25 comprises a negative uniaxial medium 22 the fastextraordinary axis of which is oriented along the normal 34 to the filmand the upper face 35 of which has been rubbed in the direction F21;this film also comprises a negative uniaxial medium 21 the extraordinaryn of which is inclined with respect to this normal 34 by an angle λ inthe plane containing the normal 34 and the rubbing direction F21. Theindex ellipsoid which characterizes the medium 21 is axisymmetric aboutthe axis n which is the optical axis or extraordinary axis of thismedium. An ordinary axis p is both in the XY plane of the cell andperpendicular to the extraordinary axis n, a second ordinary axis q isperpendicular to the axes n and p. A wave arriving perpendicular to thesubstrate 21 along the straight line 34 sees two neutral lines: the slowaxis is the p axis and the fast axis is the projection of the q axisonto the plane of the substrate.

When the cell of FIG. 2 is seen from the front, i.e. in the negativedirection of the Z axis, the polarizer 28 is in the foreground and theorientations of the different elements of the stack are illustrated byFIG. 4 in the XY plane of the cell. An orientation in the plane is shownin FIG. 4 by an arrow, and the orientations of the superimposed elementsare also superimposed. Thus the alignment directions A31 and A33 on thefaces of the nematic crystal layer are perpendicular, the passdirections P26 and P28 of the polarizers are also perpendicular to eachother and the rubbing directions F21 and F23 of the compensation filmsare also perpendicular to each other. The pass axis P26 of the polarizer26 is perpendicular to the direction F21 and parallel to the directionof the p axis of FIG. 3. Also, the pass axis P28 of the polarizer 28 isperpendicular to the direction F23 of the closest inclined negativeuniaxial material. Note, in FIG. 4, the internal bisector G of thealignment directions (A31, A33) of the outermost molecules of the liquidcrystal layer 20.

FIG. 5 shows an embodiment of the invention from the same point of viewas FIG. 4. In this embodiment, the liquid crystal cell comprises thestacked elements described previously, such as the cell of the prior artof FIG. 2, but the invention consists in orienting them in a differentway, which has the advantage of improving the observation angle of apanel comprising such cells.

By observing this cell according to the invention from the front, in thenegative direction of the Z axis normal to this cell, the lattercomprises successively, between a polarizer 28 of pass axis P28 at thefront and a polarizer 26 of pass axis P26 at the rear, a compensationfilm 27 comprising a first negative uniaxial material 24 with an axisalong Z and a second negative uniaxial material 23 with an axis inclinedwith respect to Z and with respect to the XY plane, the orientation ofthe second material being given in the XY plane by a direction F23; anematic liquid crystal layer 20 having an alignment direction at thefront A33 and another at the rear A31; a compensation film 25 comprisinga third inclined negative uniaxial material 21 whose orientation in theXY plane is given by a direction F21 and a fourth negative uniaxialmaterial 22 with an axis Z. The direction F23 of the second material inthe XY plane is aligned with the orthogonal projection onto this planeof the direction of the extraordinary axis n of the second negativeuniaxial material 23 with inclined extraordinary axis, and the directionF23 is oriented in the opposite direction to this projection. Similarly,the direction F21 is aligned with the projection onto the XY plane ofthe direction of the extraordinary axis n of the third negative uniaxialmaterial 21 inclined to the normal to the XY plane and in the oppositedirection.

This cell preferably has a twist of 90 degrees for the helix of thenematic crystal in the rest state, and has perpendicular alignmentdirections A31 and A33. When the liquid crystal cell is not activated,the molecules which are in a plane which is parallel to the XY plane andis situated in the middle of the thickness of the liquid crystal layer20 are approximately parallel to the Y axis. The Y axis is preferablyoriented from the bottom toward the top of the screen. The polarizationdirections P26 and P28 are uncrossed, they are separated by an angle of90+A degrees. The value of the angle A being preferably greater than 2degrees and less than 20 degrees, for example between 2 and 6 degrees.In the embodiment of FIG. 5, the polarization direction P26 at the rearof the cell is separated by an angle 90+A/2 degrees from the closestalignment direction A31 and similarly for the corresponding directionsof the front of the cell, P28 and A33, are separated by the same angletaken in the reverse direction. The bisectors of the alignmentdirections A31 and A33 (which alignment directions are, for example,rubbing directions of the substrates containing the liquid crystal) ofthe outermost molecules of the liquid crystal layer are preferably thebisectors of the pass directions of the polarizers P26 and P28.

In this embodiment of the invention, the orientation F23 in the XY planeof the inclined film 23 at the front of the cell remains parallel to thepass direction P26 of the polarizer furthest away. Similarly theorientation F21 of the inclined film 21 at the rear of the cell isparallel to the pass direction P28 of the polarizer at the front of thecell which is the furthest away from the film 21. The compensation filmsare uncrossed by the same value as the polarizers.

However, in another embodiment of the invention, the orientations F23and F21 can be uncrossed without being parallel to the pass direction ofthe corresponding polarizers.

The orientations F23 and F21 are, for example, separated by an angle90+B degrees where the angle B is nonzero and may be different from theangle A. The bisectors of the alignment directions A31 and A33 of theoutermost molecules of the liquid crystal layer within its thickness arepreferably the bisectors of the orientations F23 and F21 of the opticalcompensation structures. The angle B is preferably equal to the angle A.

In another embodiment of the invention, the uncrossing of the polarizersP26 and P28 corresponds, with reference to the initial position of FIG.4, to a rotation by a first angle C about the normal to the cell of oneof the polarizers, P26, and to a rotation by a second angle D about thesame axis but in the reverse direction of the other one of thepolarizers, P28. The polarization directions P26 and P28 are separatedby an angle 90+C+D degrees and the values of the first and of the secondangle of rotation are different. The bisectors of the alignmentdirections of the outermost molecules of the liquid crystal layer arenot collinear with the bisectors of the pass directions of thepolarizers (P26, P28). Each of the values C, D of the angles of rotationis preferably positive and less than 20 degrees. The two angular valuesC and D are not simultaneously zero. Each compensation structure ispreferably uncrossed like the closest polarizer. In a preferredembodiment, one of the two angular values of rotation is zero, forexample the first angle C is zero. In this embodiment shown in FIG. 6, afirst compensation structure is not uncrossed on one side of the liquidcrystal layer, for example the orientation structure F21 on the incidentilluminating light side, and a second orientation compensation structureF23 is uncrossed. In the implementation of this embodiment, only thesecond uncrossed orientation compensation structure F23 needs to be cutfrom the material of a commercial structure, taking the uncrossing angleinto account, the first structure being one used directly as soldcommercially. This embodiment is easier to produce and less expensivethan that comprising two structures modified on the basis of theavailable structures. The orientations F21 and F23 are separated by anangle 90+D. Preferably the angle C is zero and the angle D is equal to 4degrees.

In the invention, for a liquid crystal cell having polarizers of thecrossed type, the pass directions of the polarizers are no longerperpendicular. Compared with the positioning of the prior art, where thepass directions of the polarizers are effectively perpendicular, so thatone polarizer is able to block the light polarized by the other, thepolarizers according to the invention have undergone rotations about thenormal Z to the XY plane of the cell, the rotation for one of thepolarizers being in the opposite direction to that for the otherpolarizer. The polarizers of the invention are uncrossed. The uncrossingof the polarizers is measured by the value of the angle A. Theuncrossing corresponds to an angle A greater than 2 degrees. In theinvention, the structures for compensating for the contrast variationsof the liquid crystal cell are also uncrossed with each other. Eachcompensation structure 25, 27 has a direction F21, F23 in the XY planeof the cell and these two directions are not perpendicular to eachother.

Within the thickness of the cell according to the invention, therelative orientations of the twisted crystal layer, of the pair ofpolarizers and of the pair of compensation structures maximize thehorizontal viewing angle of the device. In the embodiment of FIG. 5, theuncrossing is done so as to move away from the internal bisector of thealignment directions A31 and A33 of the outermost molecules of theliquid crystal layer. The internal bisector is parallel to the Y axis.Observation of the panel is good for any observation angle within anextended range around the normal to the panel. Good observation isquantified here by the perception of a contrast at least greater than aminimum acceptable value over the angular range in question and by anabsence of contrast inversion, the contrast being the ratio of thetransmission of a cell in the activated state to that in the unactivatedstate.

In the embodiment of FIG. 6, neither the polarizer P26 nor theorientation compensation structure F21 closest to this polarizer isuncrossed. The polarizer P28 and the orientation structure F23 areuncrossed by the same value. This makes it possible to extendhorizontally the conoscope corresponding to FIG. 4, which alreadyexhibits good characteristics in the vertical direction.

The invention makes it possible to extend the range of horizontalviewing angles for which a vertical panel exhibits good legibility. Inparticular, a contrast at least equal to 40 is obtained for a horizontalviewing angle up to 40 degrees with respect to the normal to the panel.

Further, the invention allows a greater increase in the horizontalviewing angle when the uncrossing, proportional to the angle A, isgreater. For example, for an uncrossing of 2 degrees (therefore A isequal to 4 degrees), a horizontal viewing angle range of −45 degrees to+45 degrees is obtained, the range being centred on the normal to thepanel. The invention makes it possible to match the viewing angle toneed by adjusting the angular value of the uncrossing of thecompensation structure and of the polarizer.

In a addition, an alternative embodiment of the invention consists inreducing the thickness of the liquid crystal layer. For example, thethickness is reduced from 4.9 microns to 4.6 microns. For an identicaluncrossing, this alternative exhibits an improvement in the horizontalviewing angle.

What is claimed is:
 1. A display device having a liquid crystal cell, comprising: a layer of twisted nematic liquid crystal placed between two polarizers and comprising, between the layer of liquid crystal and each of the polarizers, a first compensation structure and a second compensation structure, respectively, for compensating for cell contrast variations according to an observation angle, wherein each of the first and second compensation structures comprise (1) a layer of a negative uniaxial birefringent material having an extraordinary axis perpendicular to a plane of the cell, on which (2) a layer of a negative uniaxial birefringent material having an extraordinary axis inclined with respect to a normal to the plane of the cell is superimposed, respective orientations of the first and second compensation structures being in the opposite direction to respective projections of the inclined extraordinary axes to the plane of the cell; the pass directions of the polarizers are separated by an angle of 90+A degrees, wherein A is not zero; and the respective orientations of the first and second compensation structures are separated by an angle of 90+B degrees, wherein B is not zero.
 2. The device as in claim 1, wherein the twist is at 90 degrees and the angle A is greater than 2 degrees.
 3. The device as in claim 1, wherein the bisectors of the alignment directions of the outermost molecules of the liquid crystal layer are the bisectors of the pass directions of the polarizers.
 4. The device as in claim 1, wherein the bisectors of the alignment directions of the outermost molecules of the liquid crystal layer are the bisectors of the orientations of the first and second compensation structures.
 5. The device as in claim 1, wherein bisectors of the alignment directions of the outermost molecules of the liquid crystal layer are not collinear with the bisectors of the pass directions of the polarizers.
 6. The device as in claim 1, wherein the directions of the polarizers and the orientations are separated by the internal bisector of the alignment directions of the outermost molecules of the liquid crystal layer.
 7. The device as in claim 5, wherein twisting of the polarizers corresponds to a rotation by a first angle C about the normal to the cell of one of the polarizers, and to a rotation by a second angle D about the same axis but in the opposite direction to the other of the polarizers, the polarization directions are separated by an angle 90+C+D degrees and the values of the first angle C and the second angle D of rotation are different.
 8. The device as in claim 7, wherein one of the first and second angles C and D is zero, the other being nonzero.
 9. The device as in claim 8, wherein the compensation structure closest to each of the polarizers is twisted by the same value as the polarizer.
 10. The device as in claim 8, wherein the nonzero angle is equal to four degrees.
 11. The device as in claim 1, wherein, within a thickness of the cell, orientations of the twisted crystal layer, the pass directions of the polarizers, and the respective orientations of the first and second compensation structures maximize a horizontal viewing angle of the device.
 12. The device as in claim 1, wherein the angles A and B are equal.
 13. The device as in claim 1, wherein the value of the angle A and the angle B are between 2 and 6 degrees.
 14. The device as in claim 1, wherein an increase in a horizontal viewing angle is adjustable by increasing a twisting of the polarizers.
 15. The device as in claim 1, wherein a horizontal viewing angle of the device is increased by reducing a thickness of the liquid crystal layer.
 16. A display device having a liquid crystal cell, the liquid crystal cell comprising: a layer of twisted nematic liquid crystal placed between first and second polarizers, a first compensating structure disposed between the layer of twisted nematic liquid crystal and said first polarizer arranged to receive light from a light source, and a second compensating structure disposed between the layer of twisted nematic liquid crystal and said second polarizer arranged away from the light source, wherein each of said first and second compensating structures includes a layer of a negative uniaxial birefringent material having an extraordinary axis perpendicular to a plane of the cell, said layer of the negative uniaxial birefringent material superimposed with a layer of a negative uniaxial birefringent material having an extraordinary axis inclined with respect to a normal to the plane of the cell; said second compensating structure is oriented in a direction opposite to a projection of the inclined extraordinary axis to the plane of the cell; pass directions of the first and second polarizers are separated by an angle of 90+A degrees, wherein A is not zero; and orientations of said first and second compensating structures are separated by an angle of 90+B degrees, wherein B is not zero.
 17. A display device having a liquid crystal cell, the liquid crystal cell comprising: a layer of twisted nematic liquid crystal placed between first and second polarizers; a first compensating structure disposed between the layer of twisted nematic liquid crystal and said first polarizer, and a second compensating structure disposed between the layer of twisted nematic liquid crystal and said second polarizer, wherein each of said first and second compensating structures includes a layer of a negative uniaxial birefringent material having an extraordinary axis perpendicular to a plane of the cell, said layer of the negative uniaxial birefringent material superimposed with a layer of a negative uniaxial birefringent material having an extraordinary axis inclined with respect to the normal to the plane of the cell; an orientation of said negative uniaxial birefringent material of said second compensating structure is parallel to a pass direction of said first polarizer; each of said compensating structures is oriented in respective directions opposite to respective projections of the inclined extraordinary axes to the plane of the cell; the pass direction of said first polarizer and a pass direction of said second polarizer are separated by an angle of 90+A degrees, wherein A is not zero; and orientations of said first and second compensating structures are separated by an angle of 90+B degrees, wherein B is not zero. 